The present invention relates generally to circuitry for electronic ballasts for use with fluorescent lamps, and more particularly to an electronic ballast designed to extend the the life of lamps by operation of a power controller applying two complementary high frequencies, a first frequency for filament pre-heating under fixed voltage control and a second complementary frequency, applied after a frequency adjustment-based transition period, for lamp ignition.
It is known in the prior art to use a ballast circuit to heat the two filaments of a fluorescent lamp to a high temperature, such that when an electric field is applied between the filaments, they more easily emit electrons and ionize the gas in the lamp. Responsive to radiation generated due to the electric current flowing through the gas, phosphors coating the inner surface of the lamp fluoresce, emitting visible light. The ballast typically controls both the initial ignition and the steady-state operation of the lamp.
Electronic ballasts apply this pre-heating circuitry prior to lamp ignition to lengthen lamp filament life, and thus lamp life, by increasing the concentration of electrons with a sufficient energy level to be discharged from the metal filament when a starting or striking voltage is applied to it. Typically, prior art pre-heating circuits operate using a current control technique, to maintain the filament heating current at a constant value. The resulting filament power dissipation which results is according to the standard equation, P=I2R where P is the power dissipated in the filament, I is the constant current, and R is the filament resistance.
When the lamp is new, the cold resistance of the filament is significantly lower than its value after the lamp has been in use over a period of time. Thus, according to the filament power dissipation equation above, the ageing of the filament causes it to have an increasing power dissipation. The lamp dims over time as this increasing power dissipation causes the filament to deteriorate more rapidly, until finally the filament breaks and the lamp ceases to ignite.
Examples of electronic ballasts incorporating current-controlled pre-heating circuitry include U.S. Pat. No. 5,656,891 to Luger et al., which discloses a continuously variable heating power, and U.S. Pat. No. 5,500,576 to Russell, which discloses a predetermined lamp warm-up time using current control. PCT Pat. Appln. publication WO 97/13391 discloses the use of low-voltage windings of a transformer to supply sufficient current to pre-heat the filaments.
It is also known in the prior art of electronic ballast design to provide a lamp driving circuit which operates at one frequency during the pre-heating phase of lamp, and at a different steady-state operating frequency. In U.S. Pat. No. 4,553,071 to Boyd, a ballast is disclosed having a tuned circuit which limits the current during warm-up, and when the lamp filament resistance increases, the tuned circuit develops a starting voltage for lamp ignition.
U.S. Pat. No. 5,686,798 to Mattas provides a single driving signal frequency, as contrasted with ballasts which operate at more than one frequency and use feedback circuitry to sense when lamp ignition has occurred so as to determine when to switch between the frequencies.
U.S. Pat. No. 4,641,061 to Munson discloses a ballast operating at a selected frequency high enough to develop a starting voltage, while being below the resonant frequency of the LC circuit coupled to the lamp. After starting conduction through the lamps, the frequency is reduced to a frequency substantially below the selected frequency, to limit the current flow.
U.S. Pat. No. 5,021,714 to Swanson et al discloses a circuit for starting and operating fluorescent lamps from an AC low-frequency power source. A ballast generates a voltage, whose frequencies include a plurality of harmonics of the power-source frequency, which voltage causes a capacitor and a cathode heating transformer to resonate responsive to the harmonics. The resonant voltage is applied across the fluorescent lamps to aid the starting of their discharge, and thereafter the lamps operate at the AC power source frequency.
U.S. Pat. No. 5,723,953 to Nerone et al., discloses a high voltage gas discharge lamp ballast, including a resonant load circuit which incorporates the lamp, and includes two resonant impedances whose values determine the operating frequency of the resonant load circuit. High voltage switches are used to disconnect the lamp""s filaments during the pre-heating phase.
U.S. Pat. Nos. 5,208,511 and 5,175,470 to Garbowicz, disclose a fluorescent lamp system which includes a ballast with primary and secondary windings and a switch for each electrode of each of the lamps in the lamp system. Each switch operates in response to the voltage across its associated lamp, such that after the lamp turns on, the switch interrupts the connection of its associated electrode to a heater winding.
Additionally, U.S. Pat. No. 5,015,923 to Nilssen, U.S. Pat. No. 5,563,473 to Mattas et al., and U.S. Pat. No. 5,677,602 to Paul et al., describe other electronic ballasts for use with fluorescent lamps.
As described above, lamp operation is here comprised of a filament pre-heating phase and a lamp ignition phase. In the lamp ignition phase, which follows the filament pre-heating phase, a relatively high voltage is placed across the lamp. Prior art electronic ballasts which rely on a switching arrangement to handle the transition between these phases, such as the pre-conditioner of the above-mentioned Mattas patent, do not account for the actual filament conditions obtaining after the pre-heating phase. That is, if there has been sufficient filament pre-heating, a relatively low starting voltage across the lamp is all that is needed to ignite the lamp and the filament is thereby not overly stressed. Whereas, if the filament pre-heating has been less than sufficient, then a typically higher starting voltage will be required to ignite the lamp, and this higher starting voltage of existing ballast designs will develop a stress on the filaments during ignition, thereby shortening the life of the filament, and thus the life of the lamp.
Once a lamp filament has been broken, the useful life of the lamp is effectively over. The application of starting voltage to such a lamp may be hazardous, as dangerously high voltages may be produced at the lamp socket terminals. In U.S. Pat. No. 5,747,941 to Shackle et al., an electronic ballast is disclosed which prevents a starting cycle if the lamp filaments are not intact.
As can be seen from the above, in order for electronic ballast design to extend lamp life, it must (1) overcome the problems related to constant current control as a means of filament pre-heating, since the resulting filament power dissipation will increase as the lamp ages, and (2) it must provide a transition to lamp ignition in such a way that prevents premature application of the lamp ignition voltage, before completion of the pre-heating phase.
Therefore, it would be desirable to overcome the above-mentioned problems related to pre-heating and ignition associated with existing electronic ballast designs, and provide an efficient, compact and inexpensive electronic ballast design capable of extending lamp life.
It is an object of the present invention to provide an improved ballast circuit for use in operating a fluorescent lamp in order to extend lamp life.
It is another object of some aspects of the present invention to provide improved devices and methods for pre-heating, igniting, and maintaining efficient steady-state operation of a fluorescent lamp.
It is a further object of some aspects of the present invention to provide improved devices and methods for generating a smooth transition between the pre-heating phase, the ignition phase, and the steady-state phase of fluorescent lamp operation.
In accordance with a preferred embodiment of the present invention, there is provided an electronic ballast for providing electrical energy to one or more fluorescent lamps having electrical discharge filaments, said ballast comprising:
a pre-heating circuit having a first resonant frequency, coupled to pre-heat the filaments;
an ignition driver circuit having a second resonant frequency, coupled to ignite an electrical discharge through a gas between the filaments; and
power controller circuitry, which provides power to the pre-heating and ignition driver circuits in succession so as to ignite the one or more lamps by first providing power in a constant voltage configuration to the pre-heating circuit substantially at the first resonant frequency in a pre-heating phase and subsequently providing power to the ignition driver circuit substantially at the second resonant frequency in an ignition phase.
In preferred embodiments of the present invention, an electronic ballast is provided for at least one fluorescent lamp, comprising two tuned resonant circuits, a pre-heating circuit and an ignition driver circuit. The two resonant circuits resonate at substantially different respective resonant frequencies, F1 and F2, responsive to a voltage signal generated by a signal generator portion of a power controller circuit. The voltage signal preferably has, at any given time, substantially only one frequency component, so that the first and second resonant circuits do not resonate simultaneously. This design insures that pre-heating is completed before the ignition signal can be applied.
Resonance of the first resonant circuit preferably causes a relatively high xe2x80x9cpre-heatingxe2x80x9d voltage to be generated in parallel across filaments of the lamp. This voltage drives current through the filaments in order to cause resistive heating of the filaments. Preferably, during this period of resonance, the voltage across the lamp (as distinguished from the voltage across each of the filaments) is maintained at a relatively low level, in order to prevent pre-ignition of the lamp. The signal generator typically continues to output the signal at F1 (the frequency corresponding to the resonant frequency of the first resonant circuit) while the filaments are increasing in temperature.
The voltage across the filaments is controlled so as be relatively constant. In accordance with the principles of the invention, the use of voltage control instead of current control as in the prior art, greatly increases lamp life. Since it is known that as the filaments age their resistance goes up, then according to the equivalent filament power dissipation equation for voltage, P=V2/R, since V is now constant, the power dissipated in the filament decreases over time, which is turn greatly increases the life of the filament, and hence of the lamp, compared to constant current control techniques.
When the filaments have reached a temperature suitable for ignition of gas within the lamp, output of the signal generator preferably smoothly changes from F1 to F2, in order to: (a) substantially terminate resonance in the first circuit and thereby reduce the voltage which causes heating of the filaments; and (b) initiate resonance in the second circuit, causing a large voltage drop across the lamp, thereby causing electron-discharge which develops a current flowing between the filaments in order to ignite the gas within the lamp. The time interval for preheating is preset so as to insure sufficient filament preheating has taken place before the ignition signal is applied.
Thereafter, the signal generator preferably continues the smooth change in its output frequency to a third frequency, F3, which is relatively close to F2, but relatively far from F1, in order to begin a steady-state operational phase of the ballast, characterized by: (a) provision of current necessary to operate the lamp; and (b) improved efficiency relative to ballasts known in the art, due to relatively low power losses from the filaments during steady-state operation. Additional lamp life is also achieved by minimizing filament power dissipation during steady-state operation of the lamp.
The ballast of the present invention thus differs from ballasts known in the art (e.g., U.S. Pat. No. 5,208,511, described hereinabove) which use switches to control pre-heating and ignition and do not use two respective resonant circuits to perform these functions. By using at least two resonant circuits with respective resonant frequencies, which are driven to resonate at different times responsive to a control signal for pre-heating, ignition, and steady-state operation of one or more fluorescent lamps, ballasts in accordance with the present invention can be made generally less costly and more reliable than ballasts known in the art.
In some preferred embodiments of the present invention, the ballast supplies voltage to pre-heat, ignite, and support the steady-state operation of two or more fluorescent lamps. Preferably, the two or more lamps are connected in series, and the filaments therein are connected in parallel. Further preferably, the filaments are pre-heated in parallel, and current flows in series through the lamps during the ignition and steady-state phases.
Preferably, the voltage drop across the lamps (as distinguished from the drop across the filaments therein) is maintained at a low level during the pre-heating phase, in order to prevent pre-ignition, i.e., ignition of the lamps prior to the attainment of an appropriate filament temperature. As discussed above, pre-ignition damages filaments, thereby reducing the life-span of fluorescent lamps.
Further preferably, the flow of electrons through the filaments (but not through the ionized gas), which is maintained at a high level during the pre-heating phase, is substantially reduced during steady-state operation, resulting in reduced electric power consumption and longer filament life and hence lamp life.
Preferably, the pre-heating circuit is coupled to the filaments in parallel. Further preferably, the ballast provides energy to two or more fluorescent lamps, such that the ignition driver circuit is coupled in series across the filaments of the two or more lamps.
In the preferred embodiment, the ballast design as described further herein, with lamps in series across the ignition driver circuit and filaments in parallel across the pre-heating circuit, is unique in that even with a broken filament, sufficient ignition voltage will be placed across the lamps to ignite them. This is true in the case of a single broken filament, where there is partial pre-heating by the second filament, enabling the electron-discharge. It is also true in many cases with both filaments broken. This feature further extends lamp life.
In a preferred embodiment, the power controller circuitry acts as a driver which smoothly varies the frequency at which it provides power from the first resonant frequency to the second resonant frequency in order to terminate pre-heating and initiate ignition.
Preferably, the power controller circuitry, subsequent to ignition, varies the output frequency to a third frequency, in order to drive current through the gas and cause the one or more lamps to emit light. Further preferably, the magnitude of the current driven at the third frequency is lower than the magnitude of the current driven at the second frequency.
Preferably, when the power controller circuitry provides the power at the first resonant frequency, the voltage drop generated by the ignition driver circuit between the filaments is less than an ignition threshold of the one or more lamps.
In a preferred embodiment, after ignition of the one or more lamps, energy generated by the pre-heating circuit that is dissipated by the filaments is substantially less than energy generated by the ignition driver circuit that is dissipated in the gas between the filaments.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for providing electrical energy to one or more fluorescent lamps having filaments, including:
generating a driving current at a first frequency to pre-heat the filaments of the one or more lamps; and
changing the driving current to a second frequency in order to ignite an electrical discharge between the filaments within the one or more lamps.
Preferably, generating the driving current at the first frequency includes generating a resonant current flow in pre-heating circuitry coupled to the one or more fluorescent lamps in order to drive current through the filaments.
Further preferably, generating the driving current at the second frequency includes generating the flow of a resonant current in ignition driver circuitry coupled to the one or more fluorescent lamps in order to drive current through gas between the filaments in the one or more lamps.
In a preferred embodiment, changing the driving current includes smoothly modulating the frequency of the driving current from the first frequency to the second frequency.
The smooth transition of the frequency from the first to the second frequency causes ignition of the lamps, when the appropriate voltage is developed across the second resonant circuit comprising the ignition driver circuit. Since the pre-heating circuit has sufficiently heated the filaments, this voltage does not apply an excessive stress to the filaments, thereby extending the lamp life.
Preferably, the driving current is changed from the second frequency to a third frequency in order to drive current through the gas and cause the one or more lamps to emit light. Further preferably, the magnitude of the current driven at the third frequency is lower than the magnitude of the current driven at the second frequency.
Still further preferably, driving the current at the first resonant frequency includes providing energy to the one or more lamps such that the voltage drop generated by the ignition driver circuit between the filaments is less than an ignition threshold of the one or more lamps.
In a preferred embodiment, changing the current to the second frequency includes providing energy to the one or more lamps such that after ignition thereof, energy generated by the preheating circuit that is dissipated across the filaments is substantially less than energy generated by the ignition driver circuit that is dissipated in the gas between the filaments.
As mentioned above, advantages of the inventive electronic ballast in extending the lamp life are due to the voltage-controlled pre-heating circuit, and the smooth operating frequency transition from pre-heating to ignition. In addition, the inventive electronic ballast is capable of igniting the lamps even in the case of broken filaments. Other features and advantages of the invention will become apparent from the following drawings and description.